1. Field of the Invention
The present invention relates to a display device and a method for controlling the backlight thereof.
2. Description of the Prior Art
Generally, a display device, especially a liquid crystal display (LCD) apparatus, is provided with a backlight device functioning as a light emitting device to provide images sufficient brightness such that the visibility of the display device can be enhanced under dim lighting conditions. Furthermore, relevant techniques have been developed to control the illuminance of the backlight device based on the ambient lighting condition, thereby the visibility of the LCD apparatus can be improved, allowing devices having LCD apparatus, such as mobile phones, cameras and personal digital assistances (PDAs), to be used under a wide range of lighting conditions.
The Japanese Patent Published No. 2005-24796 discloses a backlight constructed as a multilayer structure. In this case, the backlight is controlled by detecting brightness of surroundings of the liquid crystal display device with a photosensor.
The range of brightness of the detected external light may be quite wide (e.g. in a range from 10 to 10000 lux), thus it is impossible to cover the entire range of light intensity with an output level only. Therefore, the photosensor must switch among a plurality of sensitivity ranges to output the most appropriate signal range so as to obtain the signals.
FIG. 1 is a schematic view showing the structure of a conventional LCD apparatus that utilizes the output of a photosensor to control the backlight thereof according to the prior art. The backlight device 1 is disposed at the bottom of the structure, and the photosensor 11 is disposed on a glass substrate 10 of the LCD panel to generate corresponding light intensity signals based on the detected light intensity. The selection and measurement device for sensitivity range 12 is constructed by four standard switches SW1-SW4 to select the light intensity among the four sensitivity ranges so as to output the most appropriate sensitivity range. In order to select a most appropriate sensitivity range, the switches have to operate to perform the switching among various sensitivity ranges as follows, so as to determine within which sensitivity range the current signal falls.
The control device 20 is configured to access the outputted sensitivity range and to control the illuminance of the backlight based thereon.
When the backlight is turned on, the photosensor will check the light intensity and thus fail to accurately measure the intensity of the external light. In this case, the backlight has to be turned off to accurately measure the external light.
FIG. 2 is a flowchart showing the process of selecting the most appropriate sensitivity range according to the prior art. In such process, four sensitivity ranges of different levels A, B, C and D are used to cover the entire sensitivity range of the external light, so as to determine that the level of the external light falls within which sensitivity range at the time when each image shot (approximately 16 milliseconds) starts to be displayed.
In Step S1, the backlight is firstly turned off. As the sensitivity range A is selected, the switch SW1 is turned on and the data are being accessed. In order to access the data unaffected by the backlight, the backlight must be turned off for a period of time, e.g. 500 microseconds. Then, the process proceeds to Step S2 where the control device determines whether the accessed data fall within the sensitivity range A. If the determination in Step S2 is negative, the process proceeds to Step S3 where the backlight remains off, the switch SW2 is turned on, and the data are being accessed. After that, the process proceeds to Step S4 where a determination is made as to whether the accessed data fall within the sensitivity range B. The sensitivity ranges C and D will be selected sequentially and similar steps will be repeated so as to obtain correct data on the sensitivity range (Steps S5˜S8). If the control device determines that the data fall within one of the four sensitivities ranges, correct data will be obtained, and then the backlight will be turned on (Step S10). Meanwhile, if the control device is unable to determine which sensitivity range the data fall within, error data will be obtained, but still the backlight will be turned on (Step S9). The aforementioned steps are repeated after a duration of image phase passes (Step S11).
As the conventional LCD apparatus adopts four sensitivity ranges, the backlight will be turned off for 500×4=2000 microseconds during each image shot until the most appropriate data are obtained. Even if the most appropriate sensitivity range is obtained and the process of determining whether the data fall within the rest of sensitivity ranges stops, the average turn-off period of the backlight still lasts more than 1000 microseconds.
As it takes longer time to determine which sensitivity range the data fall within in the prior art, the backlight needs to be turned off for a long period of time. This results in a decrement in the luminescent efficiency of the backlight and the brightness of the image.